Your Xero account login doesn't need to match your My Accounting Toolbox login.  You only need to use an account that has high enough privileges to access Xero.

When you login to My Accounting Toolbox for the first time you will be prompted to setup your Xero Connection.

We will direct you to Xero where you will be asked to authenticate yourself with Xero. 

Select the Organisation you wish to connect to My Accounting Toolbox and click Authorise. 

Xero will redirect you back to My Accounting Toolbox - where you will complete the Autotask part of the setup.